Enable PR bot to create upstream manifest PRs from local merge events

[ruffsl]

Context in docker-library/official-images#3890 (comment)

It would be nice to automate the chained process of triggering an upstream PR to docker-library/official-images to update those manifests upon meaning one of @osrf-docker-builder PR locally. Given that @osrf-docker-builder is controlled through the travis CI, I'm unaware of the best method to capture github merge events for triggering further github API calls for PR creation. Are there proper methods to reactively do this, or should we just rely on the merge commits to master triggering a new CI job? I'd still like to control when upstream PRs are created though, say by keywords or dialog in PR merge message.